Codeforces 743d:chloe and pleasant prizes (tree-shaped DP)

Http://codeforces.com/problemset/problem/743/D

Test instructions: The point weights for the disjoint subtree of the maximum of two, and if there are not two disjoint subtrees, the output impossible.

Thought: Before it seems to have done this type of topic Ah, know is the tree-like DP, but do not know how to ensure that two do not intersect. After looking at someone else's code,

At the time of Dfs backtracking,

1 voidDfsintUintFA) {2Sum[u] =W[u];3      for(inti = Head[u]; ~i; i =edge[i].nxt) {4         intv =edge[i].v;5         if(v = = FA)Continue;6 Dfs (v, u);7Sum[u] + =Sum[v];8         if(Dp[u] >-inf) ans = max (ans, dp[u] +dp[v]);9Dp[u] =Max (Dp[u], dp[v]);Ten     } OneDp[u] =Max (Dp[u], sum[u]); A}

If the first execution of the IF statement, you can guarantee that only the U node as the root, its subtree has two or more than two, otherwise it will not update ans. And this time DP "U" is still only a sub-tree currently swept the maximum weight and, plus a sub-large DP "V", so that it is guaranteed to be the largest two disjoint subtree weights and. After traversing the DP "U" must be the maximum weight of all subtrees with the root of U, and then the sum "U" update whether to include the weights of this node.

1#include <cstdio>2#include <algorithm>3#include <iostream>4#include <cstring>5#include <string>6#include <cmath>7#include <queue>8#include <vector>9 using namespacestd;Ten #defineN 200010 OnetypedefLong LongLL; A ConstLL INF = 1ll<< -; - structnode - { the     intV, NXT; -}edge[n*2]; - LL Dp[n], sum[n], w[n], head[n], tot, ans; -  + voidAddintUintv) { -EDGE[TOT].V = v; EDGE[TOT].NXT = Head[u]; Head[u] = tot++; + } A  at voidDfsintUintFA) { -Sum[u] =W[u]; -      for(inti = Head[u]; ~i; i =edge[i].nxt) { -         intv =edge[i].v; -         if(v = = FA)Continue; - Dfs (v, u); inSum[u] + =Sum[v]; -         if(Dp[u] >-inf) ans = max (ans, dp[u] +dp[v]); toDp[u] =Max (Dp[u], dp[v]); +     } -Dp[u] =Max (Dp[u], sum[u]); the } *  $ intMain ()Panax Notoginseng { -     intN; theCIN >>N; +Memset (Head,-1,sizeof(head)); A      for(inti =1; I <= N; i++) Cin >>W[i]; the      for(inti =1; I < n; i++) { +         intu, v; -CIN >> U >>v; $ Add (U, v); Add (v, u); $     } -Ans =-INF; -      for(inti =1; I <= N; i++) Dp[i] =-INF; theDfs1, -1); -     if(Ans <=-inf) puts ("Impossible");Wuyi     Elsecout << ans <<Endl; the     return 0; -}

Codeforces 743d:chloe and pleasant prizes (tree-shaped DP)